Field of the Invention
The present invention relates to a scale formed on a surface of a rotating object and used to detect a displacement or velocity.
Description of the Related Art
In a conventional method of detecting a displacement or velocity of a rotating object, a scale formed on a surface of the rotating object is used. Methods of forming the scale on the surface of the rotating object include a method of directly forming the scale on the surface of the rotating object using a laser marker. However, the formation of the scale on the surface of the rotating object involves matching, to an desired interval, an interval between a mark formed first and a mark formed last on the surface of the rotating object at a joint portion therebetween, which causes difficulties. Since the periodicity of marks is lost at such a joint portion of the scale, the displacement or velocity of the rotating object cannot be correctly detected, and the rotating object cannot be provided with a drive control at high accuracy.
Japanese Patent Laid-open No. 2005-345359 discloses a displacement measuring apparatus that controls the rotating object to be in a desired drive state by using a dummy control signal at the joint portion.
In the displacement measuring apparatus disclosed in Japanese Patent Laid-open No. 2005-345359, the dummy signal is used at the joint portion of the scale instead of a detection signal. Thus, with the configuration disclosed in Japanese Patent Laid-open No. 2005-345359, the displacement or velocity of the rotating object cannot be correctly detected at the joint portion of the scale, and the rotating object cannot be provided with a drive control at high accuracy.